Method of removing nickel and cobalt impurities from zinc electrolyte solutions



patented May 30, 1950 UNITED STATES 1 PATENT OFFICE METHOD OF REMOVING NICKEL AND C- "BALT IMPURITIE S FR'OIVI ZINC 'ELEOTRO LYTE SOLUTIONS Claims. 1

'This invention relat's 'to the removal or impurities present in tine electrolyte solutions prior to the recovery of zinc therefrom by the electrolytic method, and it is directed particularly to the removal of cobalt and nickel from such solu- 'tidns.

' It-is well known that impurities in the electrolyte causegreatdifficulty during the operation or plating by electrical methods, in some cases due to the impurityi-tself plating and so causing the deposit to be impure and of less value, and in other cases due tothe impurity bringing about a lowered efficiency in the plating process. Consequently great pains are taken and much expense is incurred in removing the harmful impurities so far as possible.

It has for many years been known that purification of an electrolyte may be effected by the displacement of one metal from the solution by another higher in the electromotive series and a that such a displacement may be aidedby the presence of a ine'tallow in thefelectromotive series'. Thus it is common, after iron and other metals have been precipitated by treating an electrolyte with zinc oxide or roasted zinc ore or lime while air is blown through the solution, to treat it with Zinc dust and to increase the purifying action of the zinc dust by coating it with finely divided copper. This copper is commonly formed by precipitating it from the solution to be purified by the zinc dust.

On the basis of the above generalization as to removal of one metal in solution by another metal higher in the electromotive series, and this removal being aided by the presence of a metal low in the electromotive series, one would expect that all of the impurities could be removed without serious diificulties. Experiments have shown, however, that certain elements such as cobalt and nickel, which should, from the position assigned to them in the electromotive series, easily be removed, are exceedingly diiiicult to precipitate, the difficulty becoming more pronounced as the quantity of the impurities present in the electrolyte becomes less. Thus, it has not been possible to remove more than a few milligrams per litre, say from about down to about 10 milligrams per litre, of such impurities as cobalt and nickel, and much work has been done to determine methods of removing these and other impurities which are similarly difiicult to precipitate.

It has been proposed to use arsenic with zinc dust and copper but We have found that zinc dust, copper and arsenic additions alone are of little, if any, value in removing impurities such as cobalt and iiickeland this is illustrated by the renewing experiment which was carried out with 'a sample of tankhouse electrolyte (electrolyte ready for electrolysis in the hydrometallurgical process for winning iiilc) having the following analysis:

Zinc grams per litre About 120 Copper "milligrams per litre 0.4 dO Cobalt ;C10 16.0 Arsenic do 0.04 Antimony do 0.024

Zinc dustnand copper were then added to-this electrolyte, using the normal small amounts,

namely, 5 grams per litre er zmceust and 1.4. grams per litre of topper "as copper sulphate.

Then purification tests were run on samples of :this electrolyte with {additions of arsenic (as arsenious oxide dissolved in sodium hydroxide and water, and neutralizedto methyl orange with sulphuric acid) as set "rerthbemmrne samples being agitated for one hour at 45 C. The

analyses of the filtrates from these tests were as follows:

Analysis of treated solution in milligrams per litre Test Extra Additions Cu Cd 00 As Sb 50 mg./l. Arsenic 0.5 0.2 11.0 5 0. 022 mg./l. Arsenic 0. 6 0. 2 12.0 1.5 0. 026 200 mg./1. Arsenic- 0.7 0.2 12. 6 8. 0 0. 024 300 rag/1. Arsenic 4. 5 0. 3 l2, 9 20. 0 0.028

We have discovered that surprisingly good results are obtained in the removal of impurities and particularly cobalt if small amounts of both lead and arsenic in the form of soluble compounds are present in addition to the normal small amounts of zinc dust and a soluble copper compound. The amount of lead to be added varies according to the concentration of impurities in the particular electrolyte to be purified and precise directions as to the addition required for a given degree of purification may, therefore, not be stated. However, tests made on samples of tankhouse electrolyte show that remarkably good results are obtained with as little as 25 milligrams per litre of lead and that about 100 milligrams per litre is usually sufficient to reduce cobalt and nickel present as impurities to very small amounts.

Thus 100 milligrams per litre of lead, as lead acetate, with 175 milligrams per litre of arsenic, made cobalt not detectable in a sample of tankhouse electolyte of the following analysis:

Zinc ..grams per litre about 120 Copper milligrams per litre 0.4 Cadmium do 0.3 Cobalt do 15.5 Arsenic do 0.04 Antimony do 0.024

The additions of zinc dust and copper were as set forth above and, after agitation for one hour at 45 C., the analysis of the filtrate in milligrams per litre was as follows:

Cu Gd I As Sb 1 Not detectable.

Zinc grams per litre 121 Copper "milligrams per litre 0.5 Cadmium do 0.4 Arsenic do 0.04 Antimony do 0.028 Cobalt do 21.0 Nickel do 17.0

The additions of zinc dust and copper were as set forth above and after agitation for one hour at 45 C. the analysis of the filtrate was Nickel miligrams per litre 3.0 Cobalt do 1.8

The considerable efiect oi the lead addition is apparent on comparing the above analyses with the following result of a corresponding test carried out on the same electrotype without any addition of lead. The analysis was:

Nickel milligrams per litre 9.5 Cobalt do 17.0

Other tests carried out showed that bismuth, gold, iron, cadmium, mercury, chromium, molybdemum, phosphorus, gallium, tungsten, sulphur. tellurium, germanium, selenium, nickel, palladium and platinum are amongst others, of no value along with zinc dust, copper and arsenic; indeed certain of these metals, as for example platinum, are very detrimental, since not only is cobalt not removed, but the solution is left high in copper and arsenic.

What we claim as our invention is:

1. A method of removing impurities from zinc electrolyte solutions, comprising contacting the electrolyte with small amounts of zinc dust in the presence of small amounts of soluble copper and arsenic compounds and from about 25 to about milligrams per litre of lead in the form of a soluble compound.

2. A method of removing impurities from zinc electrolyte solutions comprising adding to the electrolyte small amounts of zinc dust, copper sulphate, a soluble arsenic compound and from about 25 to 100 milligrams per litre of lead in the form of a soluble compound.

DINSMORE LAURENCE GRIFFITH. MAVIS JEAN RANKIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,336,386 Sulman et al ..Apr. 6, 1920 1,773,676 Stevens et a1 Oct. 29, 1929 1,920,442 Tainton Aug. 1, 1933 2,396,569 Grifiith et a1. "Mar. 12, 1946 

2. A METHOD OF REMOVING IMPURITIES FROM ZINC ELECTROLYTE SOLUTIONS COMPRISING ADDING TO THE ELECTROLYTE SMALL AMOUNTS OF ZINC DUST, COPPER SULPHATE, A SOLUBLE ARSENIC COMPOUND AND FROM ABOUT 25 TO 100 MILIGRAMS PER LITRE OF LEAD IN THE FORM OF A SOLUBLE COMPOUND. 